Crude oil originates from the decomposition and transformation of aquatic, mainly marine, living organisms and/or land plants that became buried under successive layers of mud and silt some 15-500 million years ago. They are essentially very complex mixtures of many thousands of different hydrocarbons. Depending on the source, the oil predominantly contains various proportions of straight and branched-chain paraffins, cycloparaffins, and naphthenic, aromatic, and polynuclear aromatic hydrocarbons. These hydrocarbons can be gaseous, liquid, or solid under normal conditions of temperature and pressure, depending on the number and arrangement of carbon atoms in the molecules.
Crude oils vary widely in their physical and chemical properties from one geographical region to another and from field to field. Crude oils are usually classified into three groups according to the nature of the hydrocarbons they contain: paraffinic, naphthenic, asphaltic, and their mixtures. The differences are due to the different proportions of the various molecular types and sizes. One crude oil can contain mostly paraffins, another mostly naphthenes. Whether paraffinic or naphthenic, one can contain a large quantity of lighter hydrocarbons and be mobile or contain dissolved gases; another can consist mainly of heavier hydrocarbons and be highly viscous, with little or no dissolved gas. Crude oils can also include heteroatoms containing sulfur, nitrogen, nickel, vanadium and others elements in quantities that impact the refinery processing of the crude oil fractions. Light crude oils or condensates can contain sulfur in concentrations as low as 0.01 W % of sulfur; in contrast, heavy crude oils can contain as much as 5-6 W %. Similarly, the nitrogen content of crude oils can range from 0.001-1.0 W %.
The nature of the crude oil governs, to a certain extent, the nature of the products that can be manufactured from it and their suitability for special applications. A naphthenic crude oil will be more suitable for the production of asphaltic bitumen, a paraffinic crude oil for wax. A naphthenic crude oil, and even more so an aromatic one, will yield lubricating oils with viscosities that are sensitive to temperature. However, with modern refining methods there is greater flexibility in the use of various crude oils to produce many desired type of products.
A crude oil assay is a traditional method of determining the nature of crude oils for benchmarking purposes. Crude oils are subjected to true boiling point (TBP) distillations and fractionations to provide different boiling point fractions. The crude oil distillations are carried out using the American Standard Testing Association (ASTM) Method D 2892. The common fractions and their nominal boiling points are given in Table 1.
TABLE 1FractionBoiling Point, ° C.Methane−161.5 Ethane−88.6Propane−42.1Butanes −6.0Light Naphtha36-90Mid Naphtha 90-160Heavy Naphtha160-205Light gas Oil205-260Mid Gas Oil260-315Heavy gas Oil315-370Light Vacuum Gas Oil370-430Mid Vacuum Gas Oil430-480Heavy vacuum gas oil480-565Vacuum Residue565+ 
The yields, composition, physical and indicative properties of these crude oil fractions, where applicable, are then determined during the crude assay work-up calculations. Typical compositional and property information obtained from a crude oil assay is given in Table 2.
TABLE 2PropertyUnitProperty TypeFractionYield Weight and Volume %W %YieldAllAPI Gravity°PhysicalAllViscosity Kinematic @ 38° C.°PhysicalFraction boiling >250° C.Refractive Index @ 20° C.UnitlessPhysicalFraction boiling <400° C.SulfurW %CompositionAllMercaptan Sulfur, W %W %CompositionFraction boiling <250° C.NickelppmwCompositionFraction boiling >400° C.NitrogenppmwCompositionAllFlash Point, COC° C.IndicativeAllCloud Point° C.IndicativeFraction boiling >250° C.Pour Point, (Upper)° C.IndicativeFraction boiling >250° C.Freezing Point° C.IndicativeFraction boiling >250° C.Microcarbon ResidueW %IndicativeFraction boiling >300° C.Smoke Point, mmmmIndicativeFraction boiling between150-250Cetane IndexUnitlessIndicativeFraction boiling between150-400Aniline Point° C.IndicativeFraction boiling <520° C.
Due to the number of distillation cuts and the number of analyses involved, the crude oil assay work-up is both costly and time consuming.
In a typical refinery, crude oil is first fractionated in the atmospheric distillation column to separate sour gas and light hydrocarbons, including methane, ethane, propane, butanes and hydrogen sulfide, naphtha (36°-180° C.), kerosene (180°-240° C.), gas oil (240°-370° C.) and atmospheric residue (>370° C.). The atmospheric residue from the atmospheric distillation column is either used as fuel oil or sent to a vacuum distillation unit, depending on the configuration of the refinery. The principal products obtained from vacuum distillation are vacuum gas oil, comprising hydrocarbons boiling in the range 370°-520° C., and vacuum residue, comprising hydrocarbons boiling above 520° C. The crude assay data help refiners to understand the general composition of the crude oil fractions and properties so that the fractions can be processed most efficiently and effectively in an appropriate refining unit. Indicative properties are used to determine the engine/fuel performance or usability or flow characteristic or composition. A summary of the indicative properties and determination methods with description are given below.
The cetane number of diesel fuel oil, determined by the ASTM D613 method, provides a measure of the ignition quality of diesel fuel; as determined in a standard single cylinder test engine; which measures ignition delay compared to primary reference fuels. The higher the cetane number; the easier the high-speed; direct-injection engine will start; and the less white smoking and diesel knock after start-up are. The cetane number of a diesel fuel oil is determined by comparing its combustion characteristics in a test engine with those for blends of reference fuels of known cetane number under standard operating conditions. This is accomplished using the bracketing hand wheel procedure which varies the compression ratio (hand wheel reading) for the sample and each of the two bracketing reference fuels to obtain a specific ignition delay, thus permitting interpolation of cetane number in terms of hand wheel reading.
The cloud point, determined by the ASTM D2500 method, is the temperature at which a cloud of wax crystals appears when a lubricant or distillate fuel is cooled under standard conditions. Cloud point indicates the tendency of the material to plug filters or small orifices under cold weather conditions. The specimen is cooled at a specified rate and examined periodically. The temperature at which cloud is first observed at the bottom of the test jar is recorded as the cloud point. This test method covers only petroleum products and biodiesel fuels that are transparent in 40 mm thick layers, and with a cloud point below 49° C.
The pour point of petroleum products, determined by the ASTM D97 method, is an indicator of the ability of oil or distillate fuel to flow at cold operating temperatures. It is the lowest temperature at which the fluid will flow when cooled under prescribed conditions. After preliminary heating, the sample is cooled at a specified rate and examined at intervals of 3° C. for flow characteristics. The lowest temperature at which movement of the specimen is observed is recorded as the pour point.
The aniline point, determined by the ASTM D611 method, is the lowest temperature at which equal volumes of aniline and hydrocarbon fuel or lubricant base stock are completely miscible. A measure of the aromatic content of a hydrocarbon blend is used to predict the solvency of a base stock or the cetane number of a distillate fuel Specified volumes of aniline and sample, or aniline and sample plus n-heptane, are placed in a tube and mixed mechanically. The mixture is heated at a controlled rate until the two phases become miscible. The mixture is then cooled at a controlled rate and the temperature at which two phases separate is recorded as the aniline point or mixed aniline point.
To determine these properties of gas oil or naphtha fractions conventionally, these fractions have to be distilled off from the crude oil and then measured/determined using various analytical methods that are laborious, costly and time consuming.
SD is a technique which separates individual hydrocarbon components in the order of their boiling points, and is used to simulate time-consuming laboratory-scale physical distillation procedures. The separation is usually accomplished with a gas chromatograph equipped with a chromatography column coated with a nonpolar (hydrocarbon-like) stationary phase, an oven and injector which can be temperature programmed with. A flame ionization detector (FID) is used for detection and measurement of the hydrocarbon analytes. The SD analysis result provides a quantitative percent mass yield as a function of boiling point of the hydrocarbon components of the sample being analyzed. The chromatographic elution times of the hydrocarbon components are calibrated to the atmospheric equivalent boiling point (AEBP) of the individual n-alkane as described in a method from the ASTM by using n-alkane (n-paraffin) reference material. In the SD method ASTM D2887, the n-alkane calibration reference covers the boiling range 55-538° C. (100-1000° F.) which covers the n-alkanes with a chain length of about C5-C44.
Alternative methods may be used, including ASTM D5236, ASTM D86, ASTM D5399, ASTM D6352-04, ASTM D7213-05e1, ASTM D7398-07, ASTM D7169-05, ASTM D7096-10, ASTM D7500-10, ASTM D5307-97, ASTM D1160, ASTM D2892, or any other methods based upon gas chromatography, true boiling point distillation, supercritical fluid chromatography, and equilibrium flash. There are well-known correlations to convert distillation numbers: for example, if the true boiling point distillation (ASTM D2892) is known, the SD data (ASTM D2892) can be estimated. While there are not yet ASTM methods using supercritical fluid chromatography, non-standardized lab methods are known.
In the high temperature simulated distillation method (HTSD), the n-alkane calibration reference (a hydrogenated polyolefin wax, polywax 655) covers the boiling range 36-750° C. (97-1382° F.) which covers the n-alkanes with a chain length of about C5-C120. A key difference between ASTM D2887 and HTSD is the ability of HTSD to handle residue-containing samples (i.e. material boiling>538° C. or 1000° F.). SD and laboratory-scale physical distillation methods are routinely used for determining boiling ranges of petroleum crude oils and refined products. The boiling points with yield profile data of these materials are used by refinery engineers to make operational decisions to improve product yields and product quality. SD is valuable for, and can improve results from, computer modeling of refining processes for improvements in design and process optimization. Precise yield correlations between HTSD and crude assay distillation (a procedure which uses methods ASTM D2892 and D5236) have allowed HTSD to be successfully used in place of the physical distillation procedures. A typical simulated distillation chart obtained from a gas chromatogram of crude oil is shown in FIG. 1, where the W % of distilled fractions is plotted against the boiling temperature.
Any new rapid, direct method to help better understand the crude oil composition and properties from the analysis of whole crude oil will save producers, marketers, refiners and/or other crude oil users substantial expense, effort and time. Therefore, a need exists for an improved system and method for determining the properties of crude fractions from different sources and classifying the crude oil fractions based on their boiling point characteristics and/or properties.